Automobiles powered by an internal combustion engines not only emit pollutant emissions via combustion of fuel or via emission of lubricant or fuel in the crankcase, they also produce hydrocarbon emissions via evaporation of fuel stored in the automobiles. It is believed that approximately 20% of all hydrocarbon (HC) emissions from the automobile originate from evaporative sources. To reduce or eliminate this form of emission, modern automobiles store the fuel vapor in a canister and control its release from the canister into the combustion chamber for combustion. Such on-board evaporative emission control system (EVAP) typically includes a charcoal type vapor canister that collects vapor emitted from a fuel tank and a vapor control valve that regulates the amount of vapor permitted to be released from the canister to the engine. The EVAP system is designed to be fully enclosed so as to maintain stable fuel tank pressures without allowing fuel vapors to escape to the atmosphere.
Fuel vapor is generally created in the fuel tank as a result of evaporation. It is then transferred to the EVAP system charcoal canister when tank vapor pressures become excessive. When operating conditions can tolerate additional enrichment, these stored fuel vapors are purged into the intake manifold and added to the incoming air/fuel mixture. The EVAP system delivers these vapors to the intake manifold to be burned with the normal air/fuel mixture. This fuel vapor from the canister is added to the combustion chambers during periods of closed loop operation of the engine when the additional enrichment can be managed by the closed loop fuel control system.
It is believed that inaccurate control of the vapor control valve of the EVAP system may cause rich driveability problems, as well as failure of the various idle speed tests or enhanced I/M evaporative pressure or purge test. That is, a determination of when to permit fuel vapor to be purged to the engine is believed to be problematic due to the wide variations in the volume of fuel vapors produced in the tank that arises from various factors such as, for example, ambient temperature, pressure, fuel mixture or the volume of fuel in the tank. Moreover, the concentration of hydrocarbons or other chemical constituents in fuel vapor may vary greatly depending on these factors. Also, the amount of latent energy stored in the fuel vapor may influence the driveability and exhaust emission of the vehicle. And inappropriate over or under purging of the vapor canister may reduce efficiency or even failure of the canister.